By the use of germfree animals we plan to continue the determination of the effect of microorganisms on the response of mammals to wholebody ionizing radiation. We are investigating the pathology of graft-vs-host (GVH) disease by specific stains and fluorescent antibodies using specific antisera. We are attempting to obviate the histopathology of GVH observed in germfree radiation chimeras by pretreatment of the bone marrow inoculum with anti theta serum and thymectomy. We are also attempting to caesarian derive the "nude" mouse, which is devoid of a functional thymus, to place it in the germfree environment to be used as a model for the study of immunological reconstitution of the immune deficient patient.